There is a need in the material and polymer sciences areas to develop polymeric materials with desired in-use performance characteristics that can be readily prepared. A particularly useful class of polymers are polyureas, including linear, branched or cross-linked polyureas. Because of their elastomeric, strength, and water-proof properties, polyureas are used in a wide range of applications, and particularly as coatings in various industrial and commercial applications. See for example, US Patent Application Publication No. 2014/0221567 A1, to Fazel et al., published Aug. 7, 2014; US Patent Application Publication No. 2012/0295104 A1, to Barker, published Nov. 22, 2014; and US Patent Application Publication No. 2011/0082274 A1, to Hansen et al., published Apr. 7, 2011; which are all incorporated by reference herein in their entirety.
Polyureas can be synthesized by combining multifunctional isocyanates and amines. The reaction is generally clean and efficient. The curing of polyureas can be very fast even at ambient conditions. However, too fast a reaction can cause processing difficulties, because cross-linking occurs before complete homogenization of the monomeric components. It is generally necessary to isolate the monomer components, which can require specialized apparatuses for the polyurea processing. For example, it is possible to use a two-nozzle spray to apply polyurea coatings, which can facilitate the mixing of the two monomer components during application. However, this method is wasteful and not user-friendly, because cumbersome personal protection equipment is needed to prevent the inhalation of toxic mists. For step-growth polymerization of polyureas, curing usually requires high monomer concentrations, because at low concentrations the mixing of multifunctional components usually yields only cyclic oligomers. However, in the present invention, we have taken advantage of this low concentration phenomenon in conjunction with certain dynamic bonds, such as hindered urea bonds (HUBs) to provide methods and systems for a facile, one-component system for making and curing polyureas.
Differing from polymers and oligomers formed with strong, irreversible covalent bonds and stable bulk properties, polymers prepared through reversible non-covalent interactions or covalent bonds exhibit various dynamic properties. The dynamic features of reversible polymers have been employed in the design of self-healing, shape-memory, and environmentally adaptive materials. However, non-covalent interactions are relatively weak, with only a few exceptions such as quadruple hydrogen bonding, high valence metal chelation, and host-guest molecular interactions. Dynamic covalent bonds, on the contrary usually have higher strength and more controllable reversibility.
The amide bond forms the basic structure of numerous biological and commodity polymers, for example nylon and polypeptides, and as such, is one of the most important organic functional groups. It has been hypothesized that the amide bond has relatively high stability due to conjugation effects between the lone electron pair on the nitrogen atom and the pi-electrons on the carbonyl p-orbital. Reversing the amide bond, i.e. amidolysis, usually requires extreme conditions, such as highly basic or acid conditions and/or high temperatures, or the presence of special reagents, such as catalysts and enzymes.
Introducing bulky substituents has been theorized to create steric hindrance to disturb the orbital co-planarity of the amide bond, which diminishes the conjugation effect and thus weakens the carbonyl-amine interaction. However, the dissociated intermediate from amidolysis, would be a ketene, and if formed would generally be too reactive to provide dynamic reversible formation of the amide bond. To make the carbonyl-amine structure reversible, it is required that the dissociated carbonyl structure be stable under ambient conditions but still highly reactive with amines. One such functional group that satisfies these requirements is the isocyanate group, which can be used to form urea linkages. Isocyanates are generally sufficiently stable under ambient conditions and can react with amines rapidly to form a urea bond, a reaction that has been broadly used in the synthesis of polyureas and poly(urethane-ureas). Therefore, it would be highly desirable to control the reversibility and the kinetics of these urea bonds in polymeric or oligomeric materials, particularly as precursors for conveniently preparing polyureas in a controlled manner from a one-component process or system.
Many currently available polymeric and oligomeric materials lack both the desired performance characteristics and dynamic properties, as it is difficult to achieve both these properties from conventional polymer technologies. For example, highly covalent cross-linked network polymers generally lack the ability to be recycled, processed and self-healed after cracks have developed. As another example, polyureas constitute an important class of polymers, however, conventional polyureas generally have a very stable bond, are not very soluble, and cannot be recycled and reshaped after polymerization.
See H. Ying et al, Dynamic urea bond for the design of reversible and self-healing polymers, J. Nature Communications, 5, 3218, published Feb. 5, 2014, and PCT Publication WO 2014/144539 A2, to The Board of Trustees of the University of Illinois, published Sep. 18, 2014, which are both incorporated by reference herein in their entirety.
As seen from the foregoing, it would be highly desirable to have improved polymers. It is apparent there is an ongoing need to develop new polymers that have both desired and controlled dynamic characteristics without compromising other in-use performance properties.
We have surprisingly found we can synthesize polyureas using a one-component process or system using cyclic polyurea oligomers having hindered urea bonds which can dynamically exchange with each other. With this chemistry, the size of the resultant polyureas can be dynamically tuned by concentration, which makes it possible to use cyclic oligomers as one-component precursors for cross-linked polyureas. Such a one-component process or system avoids the complexities and costs of two-component systems and the need to segregate the individual monomer components prior to polymerization.